1. Field of the Invention
The present invention relates to an impurity measuring method of measuring the type and amount of an impurity adhered on the surface of an object to be measured, especially, a semiconductor substrate.
2. Description of the Related Art
It is well known that when an impurity such as sodium (Na), potassium (K) or iron (Fe) is contained in a thin film, e.g., an oxide film formed on a semiconductor substrate, electrical characteristics of a semiconductor element are significantly adversely affected, even if the amount of the impurity is very small. Therefore, in order to improve the electrical characteristics of the semiconductor element, mixing of the impurity from the substrate surface must be reduced as much as possible. For this purpose, the degree of contamination on the substrate surface must be correctly analyzed and measured.
The degree of contamination on the substrate surface is conventionally measured by using secondary ion mass spectrometry, Auger spectroscopic analysis or neutron activation analysis. Since these methods require a large-scale, expensive measuring instrument, the analysis cost is high. In addition, skill is required for the analyzing operation. Also, since each analyzing method uses an electron beam or a light beam, local analysis can be performed, but the degree of contamination on the overall surface cannot be estimated.
For this reason, in place of the above instrumental analyzing methods, a method of easily measuring the degree of contamination on the overall substrate surface is proposed. In this method, an oxide film having a predetermined thickness is formed beforehand on the surface of a substrate and dissolved by using a hydrofluoric acid vapor, and the resultant solution is recovered to measure an impurity by using a spectroscopic analyzer. This method is called vapor phase cracking.
The above method, however, requires an oxide film formation step. In this oxide film formation step, an impurity is mixed from an oxidation atmosphere into an oxide film, the impurity evaporates from the substrate surface into the oxidation atmosphere, or the impurity contained in the substrate is diffused to the surface. For this reason, this method is undesirable in terms of reliability in analysis values.
In another conventional method, without forming an oxide film on the surface of a substrate by an oxidation step, the overall substrate is dipped in a hydrofluoric acid solution to dissolve a natural oxide film naturally formed on the substrate surface, and the resultant solution is recovered to measure an impurity amount by using a spectroscopic analyzer.
In this method, however, since an extremely large amount of the hydrofluoric acid solution is required for recovering the impurity, the concentration of the impurity contained in the solution is significantly decreased, and therefore analysis sensitivity and precision are degraded. In addition, according to this method, the hydrofluoric acid solution is contaminated by the impurity adhered on a vessel with very high probability.